Most welding power supplies use alternating current as their source of electricity. For many applications, such as conventional arc welding, direct current at a reduced voltage is needed. Conversion of AC to DC is commonly done in one of several ways. One way is by the use of a stepped down isolation transformer coupled to the alternating current source, a rectifying bridge and a ripple smoothing inductance. For welding operations, it is often desirable to be able to provide a constant current over a range of operating voltages. In such constant current operation, the current versus voltage curve, in which current is plotted along the X axis and voltage along the Y axis, is close to vertical over the operating range only by starting out with a high open circuit voltage. Having this high open circuit voltage always present can be a safety hazard to the operator.
Silicon controlled rectifiers (SCRs) are now commonly used to rectify and control the alternating current. The use of SCRs allows the current versus voltage curve to be truly vertical over the operating range without the need for a high open circuit voltage. This is made possible by using feedback circuitry, which senses the current flow through the electrode, in conjunction with the ability of the SCRs to conduct over only a part of each alternating current cycle. As the arc resistance varies, more or less of the alternating current is chopped (also called phased back) by the SCRs to achieve a constant average current level.
In standard welding applications, the arc voltage is relatively low, such as 20-40 volts. However, to keep the arc ionized it is necessary to apply an arc sustaining, high voltage, low current signal on top of the low voltage, high current output of the power supply. This can be accomplished using separate rectifier assemblies such as shown in U.S. Pat. No. 3,356,928 to Parrish.
To achieve something close to constant current output from alternating current power supplies, transformers, in which one of the windings is movable to change the reactive coupling between the primary and secondary windings, have been used. One problem with this type of control is that current adjustment is mechanical and therefore cumbersome and slow and not readily adaptable to feedback control.
A serious drawback with conventional low open circuit voltage power supplies is their lack of adequate short circuit protection. Short circuiting of a power supply can come about in several ways. In some welding processes, the arc is initiated by touching the electrode to the work piece, thereby creating a short circuit until the arc is struck. The transfer of molten metal from the electrode to the work piece also causes momentary short circuits during the process. Short circuiting of the power supply can occur accidentally by inadvertent connection of the output terminals. This may result from the output leads touching, a tool dropping on the output studs, or other such inadvertent means. Short circuiting may also occur in various testing or evaluation procedures. For example, Underwriters Laboratories approval of a welding power supply requires that it pass a short circuiting test.
In a constant current silicon controlled rectifier power supply using feedback control to maintain the current at a constant level, a short circuiting condition can present a very severe operating condition for the SCRs. With the arc voltage at essentially 0 volts (short circuit), the SCRs must phase back to the point where the voltage across the SCRs is equal to the voltage from the secondary of the transformer. Using conventional closely coupled transformers, the voltage across the SCRs will be close to the open circuit voltage of the transformer secondary, reduced only by the voltage drop caused by impedances of the transformer, input line, and wiring and components in the power supply itself. As a result, the SCRs must phase back to a very low conduction angle. At this low conduction angle they must be capable of handling the current that is required by the setting of the current control on the constant current power supply, possibly the maximum possible power supply output. The result is that the SCRs may see virtual current spikes, which have an average value that is within the rating of the SCRs but which have an RMS value which may exceed the rating of the SCRs. Using other terminology, the "form factor," which is defined as the ratio of the RMS value of a waveform to its average value, increases as the SCR conduction angle decreases. The form factor represents a convenient method of analysis of SCR capability. In the extreme, constant DC has a form factor of 1.0. A waveform representing a spike that approaches a zero time interval has a form factor that approaches infinity. For an SCR of a given average current rating, its capability decreases as the form factor increases, given the same average current under both conditions.
Another problem which can affect the proper operation of power supplies arises from the effect of noise or other interference in the secondary circuit. Noise is a particular problem in AC welding where there is no inductor to help filter the noise. For example, in AC tungsten inert gas (TIG) welding, a high voltage, high frequency signal is superimposed on the arc to initiate, sustain and stabilize the arc. The interference from this signal can be extremely detrimental to the operation of the SCRs if not effectively filtered. Such high frequency interference can also affect the fire control circuit regulating the SCRs.